A floristic comparison and ecological analysis of bur oak ( Quercus macrocarpa Michx.) savannas in central Nebraska

摘要

Bur oak (Quercus macrocarpa Michx.) savannas of the Great Plains have long been reported to be disappearing and/or progressing to other vegetative schemes. These losses are directly relatable to the arrival of European settlers and their subsequent suppression in natural prairie fires, along with the introduction of domestic livestock. A preliminary investigation of the floristic composition within remaining oak savannas of Custer County, Nebraska was conducted as a base means of documenting plant diversity within the remaining savannas of central Nebraska. The point-centered quarter method was used to determine species, density, relative density, frequency, relative frequency, dominance, relative dominance, and importance values (IV). The Swedish-increment boring method was applied to woody vegetation to document tree ages and compositional change in the savanna over time. Exclusion cages on oak seedlings, vegetative and soil sampling were also utilized. Results indicated a bleak future for the remaining savannas of Nebraska. Bur oak averaged an importance value of 159.10 out of 300.00, leaving a substantial portion of the importance values divided among several fire-sensitive species. The majority of bur oaks sampled were distributed among the higher size classes, indicating a low rate of recruitment. Age class distributions further support this conclusion with a minuscule number of oaks younger than 25 years of age and the majority of the species recorded over 100 years of age. Seedling survivability tests during summer months produced low survivorship (3.13%) due to drought and large/small mammal herbivory. Winter testing displayed similar results. Vegetation sampling indicated high percentages of exposed till and weedy plant species, indicating a long history of abuse. Soil tests resulted in significant differences in phosphorus and zinc concentrations. As oak recruitment failings continue on a near global scale, more research should be conducted to assist in the conservation and management of these vital ecosystems.